What does your flash need to know before it can say, "That's just enough light to do the job. I can stop now?"
(1) It needs the speed of your film. Set your ISO on the flash to match what you've got in your camera.
(2) It needs to know what is the maximum distance you want your flash to cover. Select from table on flash to match distance and f stop.
(3) Finely it needs to know you have matched your f stop on your camera to one you choose on your flash.
Now Compose, Focus and shoot. Enjoy!
In TTL it only needs ISO and in the M& the camera provides that information to the flash